Scientists said the latest rumbling fits the long-active volcano’s behavior, but they cautioned that the ash could fall for weeks and urged residents to take precautions. Government officials have said the heavy ash has already caused health hazards and could pose more.

The National Civil Protection Coordination (CNPC), Mexico’s disaster management agency, raised the volcanic threat level in its stoplight-like warning system to “Yellow Phase 3,” just shy of an evacuation order. Under the current warning phase, large lava domes can rapidly develop and explode in “increasing intensity,” launching incandescent rock and ash into the air.

The CNPC also warned of greater volcanic activity and the dispersion of ash into several nearby towns across at least three states — Mexico, Morelos and Puebla — in the center of the country.

Ash from Popocatépetl, known among Mexicans as “El Popo,” has caused disruptions in the area. Mexico City’s two main airports temporarily shut down operations Saturday, and dozens of flights were delayed afterward, according to local media; the poor air quality prompted authorities to close schools and switch to online classes in dozens of municipalities in the states of Mexico, Puebla and Tlaxcala, officials said. Hospitals and medical centers across five states have increased preparations for potential injuries.

Secretary of Public Health for the state of Puebla, Jose Antonio Martinez Garcia, said Tuesday that 165 people living near the volcano have received medical treatment because of the ash, including 133 patients with laryngitis.

About 25 million people live in a 60-mile radius of the volcano.

President Andrés Manuel López Obrador said Monday that more than 7,000 members of the military have been deployed to assist with evacuation routes, install shelters and clear out roads for fast evacuations “in case they are needed.”

Residents may feel more reassured during such eruptions, Obrador said, “than when it is silent.” He said he was in contact with federal and local officials who are closely monitoring the situation.

In case of eruption, authorities would consider evacuating 3.5 million people living in 30 municipalities in “high” and “medium” risk areas around the volcano, according to public contingency plans.

“In the case of Mexico City, the risk is ash fall,” Mayor Claudia Sheinbaum said in a tweet Sunday. “We are prepared for that scenario, and we know what to do. Let’s stay alert.”

Mexican authorities have warned residents who live near the volcano to wear masks, glasses and long sleeves to protect them from volcanic ash, to avoid exercising outdoors, to close water deposits, to seal doors and windows with damp clothes and to study evacuation routes.

The U.S. Embassy in Mexico issued a warning over the weekend, saying the Popocatépetl volcano “has exhibited increased activity since May 15, registering hundreds of tremors and smoke and ash exhalations.” It also recommended people avoid travel within a 7.5 mile radius of the volcano.

The 17,883-foot-tall, snowcapped mountain had been dormant for decades before it erupted in 1994, according to the National Disaster Prevention Center. Ever since, residents have become accustomed to rumblings and occasional spewing of ashes. In 2000, a major eruption prompted the evacuation of about 50,000 people from the region.

On Monday, scientists from the Volcanology Department at the National Autonomous University of Mexico compared the present activity to more than a dozen other episodes that have occurred since the year 1500. They said it is neither “new nor surprising.”

Since 1996, scientists have observed 86 incidents like this, in which “lava domes grow inside the volcano’s crater and disintegrate through eruptions,” said Servando de la Cruz Reyna, researcher at the Volcanology Department.

“This is not something new in the history of Mexico,” he said of recent activity at Popocatépetl , which means “smoking mountain” in Nahuatl, the Indigenous language of the Aztec civilization that is spoken in Central Mexico.

“But it is something that we are now able to witness, measure and comprehend in a better way,” he added.